Significance The long term objective of this project is to better understand the antimicrobial activity of fluoride and the mechanisms by which cariogenic bacteria become resistant to fluoride-mediated inhibition. Although fluoride therapy has been used with great success in the prevention of dental caries it has not been totally effective. Because of its implication as a prime etiological agent of human dental caries Streptococcus mutans was chosen as a model organism for study. Neither enolase activity nor binding of fluoride to cell components can account for fluoride-resistance. It has been documented that normal plaque flora may become fluoride-resistant due to daily challenge with a fluoride dentifrice. Unlike other medical treatments for disease, fluoride therapy is unique in that it is applied daily - 365 days a year. This daily regime is not used for normal antibiotic treatment in medical practice because of the likelihood of development of resistant pathogens. Because fluoride resistance is accompanied by enhanced glycogen storage and increased acid production, the resistance to fluoride by plaque bacteria may in part be responsible for the limited anti-caries efficacy of fluoride in clinical practice. The outcome of the work presented here would be a better understanding of fluoride resistance by oral bacteria so appropriate measures could be taken to increase the anti-caries activity of fluoride. Specific Aims 1. Isolation of fluoride-resistant clones of S. mutans from laboratory strains and from humans who are receiving high-dose fluoride therapy. 2. Determination of classes of fluoride-resistance will be based on minimum inhibitory and bactericidal concentrations, growth, acid production, acid resistance and fluoride-mediated cell death. 3. Determination of alternations in membrane passive permeability to protons and proton motive force. 4. Determination of changes in glucose uptake, and glycolytic activity by F-resistant clones. 5. Determination of the in vivo effect of fluoride-resistance on caries in a rat model.